The lead-acid battery is an electrochemical storage battery generally comprising a positive plate, a negative plate, and an electrolyte comprising aqueous sulfuric acid. The plates are held in a parallel orientation and electrically isolated by porous separators to allow free movement of charged ions. The positive battery plates contain a current collector (i.e., a metal plate or grid) covered with a layer of positive, electrically conductive lead dioxide (PbO2) on the surface. The negative battery plates contain a current collector covered with a negative, active material, which is typically lead (Pb) metal.
During discharge cycles, lead metal (Pb) supplied by the negative plate reacts with the ionized sulfuric acid electrolyte to form lead sulfate (PbSO4) on the surface of the negative plate, while the PbO2 located on the positive plate is converted into PbSO4 on or near the positive plate. During charging cycles (via an electron supply from an external electrical current), PbSO4 on the surface of the negative plate is converted back to Pb metal, and PbSO4 on the surface of the positive plate is converted back to PbO2. In effect, a charging cycle converts PbSO4 into Pb metal and PbO2; a discharge cycle releases the stored electrical potential by converting PbO2 and Pb metal back into PbSO4.
Lead-acid batteries are currently produced in flooded cell and valve regulated configurations. In flooded cell batteries, the electrodes/plates are immersed in electrolyte and gases created during charging are vented to the atmosphere. Valve regulated lead-acid batteries (VRLA) include a one-way valve which prevents external gases entering the battery but allows internal gases, such as oxygen generated during charging, to escape if internal pressure exceeds a certain threshold. In VRLA batteries, the electrolyte is normally immobilized either by absorption of the electrolyte into a glass mat separator or by gelling the sulfuric acid with silica particles.
One major problem with existing lead-acid batteries is their low cycleability at high rate charge/discharge conditions required for advanced applications such as hybrid electric vehicles and distributed storage. The main failure mode in these operating conditions is called “negative plate sulfation”, which is a term used to describe the phenomenon of kinetically irreversible formation of lead sulfate (PbSO4) crystallites. Ideally during each charge/discharge cycle all the lead sulfate on the negative plate is reversibly converted to lead and then back to lead sulfate. However, in reality this is not the case and during each cycle more and more lead sulfate is irreversibly formed in the negative plate. The formation of increased amounts of lead sulfate leads to several undesirable effects: the conductivity and porosity of the plate is decreased, the accessibility of sulfuric acid to the active phase is hindered and less Pb is available to participate in the discharge process, all this in combination leading to failure of the battery to deliver required voltage and power. This phenomenon is especially pronounced when fast charge/discharge cycles are used.
One known method for reducing the problem of “negative plate sulfation” is to add carbon, generally in the form of graphite, carbon black and/or activated carbon, to the paste used to produce the negative plate. The carbon increases the electrical conductivity of the active material in the discharged state thereby improving its charge acceptance. An example of such an approach is discussed in “Mechanism of action of electrochemically active carbons on the processes that take place at the negative plates of lead-acid batteries”, Pavlov et al, Journal of Power Sources, 191, 2009, 58-75, in which the effect of adding different forms of carbon at varying levels between 0.2 to 2% by weight of the negative plate paste is studied. The carbon materials investigated are NORIT AZO activated carbon and the carbon blacks VULCAN XC72R, Black Pearls 2000 and PRINTEX® XE2.
In addition, U.S. Patent Application Publication No. 2009/0325068 discloses an expander for a battery paste for a battery plate for a lead-acid battery, comprising barium sulfate; approximately 0.2% to 6% of carbon and/or graphite; and an organic material, such as a lignosulfonate.
Further, U.S. Patent Application Publication No. 2010/0015531 discloses a paste suitable for a negative plate of battery, including an activated carbon having a mesopore volume of greater than about 0.1 cm3/g and a mesopore size range, as determined by DFT nitrogen adsorption isotherm, of about 20 angstroms to about 320 angstroms
Although carbon addition is an effective approach to the reduction of “negative plate sulfation”, mechanical concerns currently limit the amount of carbon added to the negative plate paste. Thus, adding carbon requires increasing the amount of water and/or sulfuric acid in the negative paste mix to lower the viscosity of the paste. However, this often results in a reduction in the adhesion of the paste to the underlying support grid and consequently, a reduction in plate integrity during paste processing and/or plate curing. For example, the paste may be displaced from the support grid due to adhesion to the equipment used for paste processing. In addition, during plate curing, the paste may flake off the grid due to poor grid contact. Further, during the curing and/or forming of the plate, cracks may form in the electrodes which subsequently lead to poor electrode performance and poor cycleability of the batteries incorporating the electrodes.
According to the present invention it has now been found that, by using certain low structure carbon blacks, the amount of additional water and/or sulfuric acid required in the paste for the carbon addition can be substantially reduced. In this way, the amount of carbon that can be added to the paste, without the ancillary reduction in the mechanical properties of the paste and the final electrode, can be significantly increased.